With this intention, resort to polymeric materials is mandated due to their convenience of implementation, which entails the material being cured in-situ by polymerising the constituents of a previously applied monomer composition covering the products to be protected.
So that the quality of the materials obtained is suitable for encapsulating electronic components, they need to be resistant to extreme conditions of operation in terms of temperature and durability. The operating temperatures to be resisted, just as in the field of aerospace, are in the cryogenic temperature range or that of higher temperatures above 200° C., even capable of reaching 400° C.
Other qualitative requirements for these products relate to resistance to chemical agents, resistance to water or moisture and to thermal or electrical insulation capacity.
To meet these requirements the use of polymers in the polyimide class can be considered. These polymers are known for their mechanical and physico-chemical qualities in applications which however are very different from encapsulation which is the aim of the present invention. It is expedient here to refer to U.S. Pat. No. 4,526,838 and the prior art which is cited in this. The technique which is described there relates to polymers used as binder with fibrous fillers to produce composite materials.
Generally the synthesis of polyimides is carried out in solution. The presence of an aqueous base being prohibitive in the presence of electrical conductive elements of the electronic components, it is proposed in the context of this invention to carry out the curing of the polyimides in an organic solvent. On this subject, existing technical documentation describes the use of polar aprotic solvents as dissolving medium for a polymerisation reaction carried out in the presence of catalysts, which can be acid catalysts or also alkaline catalysts.
Resort to a solvent, even a non-aqueous one, as reaction medium already represents a disadvantage, especially considering the context of the application to encapsulate electronic components. In fact, the variety of electronic components which are incorporated in the same module and encapsulated assembly makes it difficult to select an organic solvent which is perfectly benign for all of them.
It is added that the traditional process of synthesising polyimides is based on a polycondensation reaction using an acid dianhydride and a diamine and that such a reaction releases water. The reaction is realised very gradually along a two stage increase in temperature curve, with a first stage of at least one day at room temperature and a second hot stage carried out at high temperature normally between 100 and 300° C., which lasts several hours. However, when this reaction takes place in the presence of electronic components as for the application of encapsulation, the water vapour released is very harmful. On the whole, such conditions are incompatible with the needs for mass-producing electronic modules manufactured on a large scale, such as those known particularly in the world of motor vehicle equipment.